It is commonly known to use a diode network to convert a triangular voltage into a sinusoidal voltage. Such a diode network approximates the sinusoidal curve by converting the increasing and decreasing slopes of the triangular waveform into suitable sections of straight lines. For a good approximation of the sinusoidal waveform a relatively large number of such sections is required leading to an expensive and complicated circuit.
In IEEE Journal of Solid State Circuits, June 1976, page 418 a sine-shaping circuit is described which uses the non-linear transfer characteristic of a transistor differential amplifier for converting a triangular voltage into a sinusoidal voltage. Although this technique allows generation of sinusoidal voltages with relatively low harmonic distortion, it has a draw-back in that the sinusoidal waveform has a bend in its amplitude at both its maximum and minimum, i.e. its first derivative is discontinuous at its maximum and minimum. The reason for this effect is that the non-linear transfer characteristic of the differential amplifier has non-zero slope at its ends since its slope only approaches zero asymptotically.
In the published German patent application 2,613,338 a circuit is described wherein the triangular signal is simultaneously fed to two differential amplifiers, whereof the one has a non-linear and the other has a linear transfer characteristic. The output signals of these two differential amplifiers are subtracted from each other. Thus, the circuit has a combined transfer characteristic with a slope which becomes zero or even negative at its ends. If the circuit is modulated exactly up to the zero slope points of the combined characteristic, a sinusoidal waveform without bends may be generated. However, in this circuit the subtraction of two signals is disadvantageous since it may lead to substantial errors as is well known from the rules of error combination.